1. Field of the Invention
This invention relates to vacuum metallising hollow bodies, in particular hollow bodies such as tap knob bodies, handles, control knobs or the like made of plastics material.
2. Description of the Prior Art
I have already devised certain improvements which are the subject of my earlier application for patent, Ser. No. 833,340 and which also relate to metallising hollow bodies. I have found that the use of a large vacuum chamber can be avoided by only reducing the interior of the hollow body to high vacuum and providing a special filament assembly by which the coating metal is vaporised adjacent an opening or mouth of the hollow body.
With my prior arrangement the production rate was limited whilst all the ancillary equipment for inducing high vacuum had to be provided, and this includes gauges, pumps, reservoirs and sequence control switches with all of the essential safety arrangements. This meant that the initial cost of the apparatus was high, and the production rate not fast enough in some installations to cope with demand and uneconomic.
Additionally, I found that certain problems arose in providing a consistent high quality coating from vaporising the coating metal within or closely adjacent to the mouth of the hollow body. Defects of finish were not acceptable where, as is usually the case, the plastics material is translucent and the metallised coating is diplayed and viewed through the body. These problems arose because variations in the design of the interior of the hollow bodies, and the fact that in many instances, the plastics material exudes impurities such as gasses during the reduction to high vacuum, and these gasses interfere with the deposition of the coating metal in an uniform and uncontaminated manner. It will be understood that the quality of the eventual coating is dependent on the pure controlled composition of the coating metal from its vaporised state.